System and method for assessing risk of glaucoma onset

ABSTRACT

A system and method for predicting the onset of glaucoma uses a Finite Element Model (FEM) to obtain a response profile of the Optical Nerve Head (ONH) inside an eye. To do this, the FEM is programmed with data from first and second images of the ONH that are respectively taken at the beginning and the end of an imposed pressure differential (e.g. over a range of about 8 kPa). The FEM is then subjected to a sequence of pressure increments and the resultant profile is compared with empirical data to predict an onset of glaucoma.

FIELD OF THE INVENTION

The present invention pertains generally to ophthalmic diagnosticsystems and methods for their use. More particularly, the presentinvention pertains to systems and methods that are used to predict theonset of glaucoma before symptoms of the disease become apparent. Thepresent invention is particularly, but not exclusively, useful as asystem or method for using a Finite Element Model (FEM) to predict theonset of glaucoma.

BACKGROUND OF THE INVENTION

Glaucoma is a medical condition where increased pressure within aneyeball causes a gradual loss of sight. Although glaucoma can not becured, if detected early enough it can be controlled by medications,surgery, or both. In any case, the important thing is to have earlydetection. In the early stages of the disease, however, there are fewdetectable symptoms that are glaucoma specific. Nevertheless, there arecertain risk factors, such as age, race, and family history, in additionto hypertension, which can indicate that an early detection (prediction)of glaucoma may be prudent. Stated differently, it may be desirable toidentify candidates early on for the pharmacological treatment ofglaucoma. And, consequently, to thereby determine a properly requiredpharmacological regimen, including the type and strength of medicationsto be used.

It is known that an increased intraocular pressure (IOP) inside theeyeball causes glaucoma. An increased IOP also causes noticeableanatomical changes in the eye. In particular, as a consequence of theincreased IOP, changes in biomechanical stress conditions in the LaminaCribrosa (LC) of the Optical Nerve Head (ONH) are observable.Importantly, these observations can be evaluated to determine whetherany damage to the LC is due to an increase in IOP. If so, glaucoma maybe indicated. On the other hand, a healthy eye, without glaucoma, willresist the cell damage that would otherwise be caused by an increase inIOP.

Anatomically, the LC is generally a cylindrical-shaped, mesh-likestructure that includes pores which pass through the structure. It islocated at the back of an eye, and is positioned in a hole through thesclera at the ONH where fibers of the optic nerve exit the eye. Inaddition to supporting these nerve fibers, it is believed that animportant function of the LC is to help maintain an appropriate pressuregradient between the inside of the eye (i.e. IOP) and the surroundingtissue. For this purpose, the LC is more sensitive to pressuredifferences than is the thicker, denser sclera surrounding the ONH.Consequently, it tends toward a measurable change in its configurationwith increased IOP. Importantly, it is believed that configurationchanges in the LC contribute to glaucoma.

Mathematical models of anatomical structures, such as components of theeye, can be very helpful diagnostic tools. In particular, whenever ananatomical structure is somehow forced to change, a Finite Element Model(FEM) is known to be helpful for evaluating the consequences of thechange. For example, U.S. patent application Ser. No. 12/205,420 for aninvention entitled “Finite Element Model of a Keratoconic Cornea” whichis assigned to the same assignee as the present invention, discloses amathematical methodology for predicting the condition of an eye inresponse to a proposed surgical procedure.

In light of the above, it is an object of the present invention toprovide a system and method for predicting the onset of glaucoma beforesymptoms of the disease become apparent. Another object of the presentinvention is to identify candidates for the pharmacological treatment ofglaucoma, and to provide information for subsequently establishing thetreatment regimen. Yet another object of the present invention is toprovide a system and method for mathematically modeling the LaminaCribrosa (LC) to create a pressure response profile for comparison withempirical data to predict the onset of glaucoma. Still another object ofthe present invention is to provide a system and method for predictingthe onset of glaucoma that is easy to implement, is simple to use and iscomparatively cost effective.

SUMMARY OF THE INVENTION

In accordance with the present invention, a system and method fordiagnosing the onset of glaucoma in an eye involves evaluatinganatomical parameters under various pressure conditions. Morespecifically, the parameters to be evaluated are associated with tissueof the Optical Nerve Head (ONH) in the eye. For this evaluation, thepresent invention relies on the use of a Finite Element Model (FEM) thatreplicates the ONH. In particular, this evaluation is based on thecomparison of an empirical statistic with a profile that is generated bythe FEM in response to a simulated pressure differential.

In detail, the FEM comprises a plurality of mathematical tensorelements, with each individual element representing anatomical tissue ata particular location on the ONH. Structurally, the FEM substantiallyreplicates the ONH as a cylindrical shaped body having a first endsurface and a second end surface, with a cylindrical surface extendingbetween the peripheries of the two end surfaces. For this configuration,tensor elements of the FEM representing the Prelaminar Neural Tissue(PrNT) are arranged on the first end surface. Elements representingPostlaminar Neural Tissue (PoNT) are arranged on the second end surface.And, between the PrNT and the PoNT, tensor elements representing theLamina Cribrosa (LC) are located inside the cylinder shape. Also, tensorelements of the FEM representing the sclera are arranged on thecylindrical surface. Further, these sclera elements include a pluralityof fiber elements that transition in an outward direction from asubstantially circumferential orientation at the cylindrical surface toan increasingly spiral orientation with increasing distance from thecylindrical surface. This is done to add stability to the FEM.

In operation, anatomical data is obtained from a patient for use inprogramming the FEM. More specifically, this acquisition of data is donein two steps. First, stress-strain measurements (data) are taken fromthe ONH when the eye is under a first pressure (e.g. 2 kPa). Thiscreates a first image of the ONH. Second, the procedure is repeated toobtain stress-strain measurements (data) when the eye is under a secondpressure (e.g. 8 kPa). This creates a second image of the ONH. Data fromthe first and second images are then programmed into the FEM.

Once the FEM has been programmed with the first and second images of theONH, the tensor parameters of the FEM are varied from a base condition(e.g. the first image) to obtain a profile of the ONH. Preferably, thisvariation covers a range of pressures (e.g. range of 8 kPa) and is donein a sequence of pressure increments, with each increment beingapproximately 1 kPa. The resultant profile is then compared withempirical data to predict an onset of glaucoma.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of this invention, as well as the invention itself,both as to its structure and its operation, will be best understood fromthe accompanying drawings, taken in conjunction with the accompanyingdescription, in which similar reference characters refer to similarparts, and in which:

FIG. 1 is a schematic of the system of the present invention shown inits relationship with an eye (shown in cross section);

FIG. 2 is an enlarged view of the Lamina Cribrosa (LC), and the OpticalNerve Head (ONH) of the eye shown in FIG. 1, and

FIG. 3 is a perspective view of a Finite Element Model presented as amathematical representation of the LC for use with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, a system for use with the presentinvention is shown and is generally designated 10. As shown, the system10 includes an imaging unit 12 that has an illumination means (notshown) for directing light along a beam path 14. Further, the system 10includes a pressure unit 16, and FIG. 1 shows that both the imaging unit12 and the pressure unit 16 provide input for creation of a mathematicalFinite Element Model (FEM) 18.

A computer 20 is shown in FIG. 1 with connections to both the FEM 18 anda database 22. As one of its functions, the computer 20 is used in thesystem 10 to run a program 24 for an operation of the FEM 18. Morespecifically, the program 24 subjects the FEM 18 to incremental pressureincreases that simulate the progress of glaucoma. For another function,the computer 20 is used to compare the output from the FEM 18 withempirical data from a database 22. Thus, the input from the FEM 18 tothe computer 20 is a consequence of the program 24. On the other hand,input from the database 22 to the computer 20 is empirical data that hasbeen clinically collected from a plethora of different patients.

As is appreciated with reference to FIG. 1, the system 10 is intendedfor use in evaluating an eye 26. More specifically, the system 10 is tobe used for evaluating the Lamina Cribrosa (LC) 28 that is located inthe Optical Nerve Head (ONH) 30 of the eye 26. The anatomical aspects ofthe ONH 30 and the LC 28 as they pertain to the present invention willbe best appreciated with reference to FIG. 2.

In FIG. 2 it will be seen that the LC 28 is surrounded by sclera 32, andincludes nerve fibers 34 that extend from the retina 36 as they exitfrom the eye 26 and into the optic nerve 38. Further, the LC 28 is amesh-like structure that includes a plurality of pores 40. Functionally,the LC 28 is continuously subjected to intraocular pressure from thevitreous body 42 of the eye 26. An FEM 18 that mathematically replicatesthe LC 28 is shown in FIG. 3.

FIG. 3 shows that a FEM 18 for mathematically representing the LC 28substantially replicates a cylindrical shaped body 44. As such the body44 has a first end surface 46 and a second end surface 48, with acylindrical surface 50 that extends between the end surfaces 46 and 48to represent the periphery of the LC 28. As intended for the presentinvention, the first end surface 46 of the body 44 is used to replicatethe location of Prelaminar Neural Tissue (PrNT) of the LC 28. And,similarly, the second end surface 48 of the body 44 is used to replicatethe location of Postlaminar Neural Tissue (PoNT) of the LC 28.

For the mathematical aspects of the FEM 18, a plethora of elements 52are arranged over the first end surface 46 of the body 44 for thispurpose (Note: the elements 52 shown in FIG. 3 are only exemplary).Also, a plethora of elements 54 (also exemplary) are arranged on thesecond end surface. Between the end surfaces 46 and 48, and within thebody 44, are elements 56 of the FEM 18 that represent the LC 28 itself.Further, fiber elements 58 that represent the sclera 32 are arranged onthe cylindrical surface 50 of the FEM 18. More specifically, these fiberelements 58 are arranged to transition in an outward direction from thecylindrical surface 50 with a transition characterized by a change froma substantially circumferential orientation at the cylindrical surface50 to an increasingly spiral orientation with increasing distance fromthe cylindrical surface 50. The purpose here is to replicate thestability provided by the sclera 32 for the LC 28. As will beappreciated by the skilled artisan, each of the elements 52, 54, 56 and58 in the FEM 18 are mathematical tensors that can be individuallyprogrammed to represent biomechanical properties of tissue at a locationin the anatomical structure being replicated.

Operation

In the operation of the system 10 of the present invention, an eye 26that is to be evaluated is subjected to a pressure differential by thepressure unit 16. More specifically, this pressure differential willpreferably be over a range of about 8 kPa. First, the eye 26 issubjected to an initial pressure (e.g. 2 kPa). With eye 26 under thisinitial pressure, the imaging unit 12 is employed to create an image ofthe LC 28. In detail, this imaging can involve well known techniquesthat include the use of confocal microscopy or Optical CoherenceTomography (OCT) for general imaging. It can also involve SecondHarmonic Generation (SHG) imaging for determining micromorphologyparameters. For instance, the location and sizes of pores 40 in the LC28 may be best determined by SHG imaging. In any event, these imagingtechniques are employed to obtain measurable data concerningbiomechanical stress/strain parameters of tissue in the LC 28. Next, theeye 26 is subjected to a subsequent pressure (e.g. 10 kPa) by thepressure unit 16. Again, while the eye 26 is under this subsequentpressure, images of the LC 28 are made and biomechanical stress/strainparameters of tissue in the LC 28 of the eye 26 are taken. All of thisinformation is then used to program the FEM 18.

Once the FEM 18 has been programmed with biomechanical stress/strainparameters taken from the eye 26, the FEM 18 is manipulated through asequence of pressure increments. More specifically, the FEM 18 is firstobserved at a base pressure, and is then subsequently observed atincreased pressure levels. These levels will typically be at intervalsof about 1 kPa. During this process, changes in the tensor parameters ofthe elements 52, 54, 56 and 58 are observed at each pressure level, andare recorded to create a pressure response profile for the eye 26.

As indicated in FIG. 1, the pressure response profile that is created asdisclosed above is provided as input to the computer 20. The computer 20is then used to compare the pressure response profile with empiricaldata retrieved from the database 22. In accordance with this comparison,it can then be determined whether the eye 26 is a glaucoma candidatethat should receive pharmacological treatment.

While the particular System and Method for Assessing Risk of GlaucomaOnset as herein shown and disclosed in detail is fully capable ofobtaining the objects and providing the advantages herein before stated,it is to be understood that it is merely illustrative of the presentlypreferred embodiments of the invention and that no limitations areintended to the details of construction or design herein shown otherthan as described in the appended claims.

What is claimed is:
 1. A method for determining an onset of glaucoma inan eye which comprises the steps of: providing a programmable FiniteElement Model (FEM) of an Optical Nerve Head (ONH) of a patient havingelements with tensor parameters respectively representative of a LaminaCribrosa (LC), Prelaminar Neural Tissue (PrNT), Postlaminar NeuralTissue (PoNT), and Sclera; measuring data of stress-straincharacteristics for tissue at each of a plurality of locations in theONH of the patient in response to a predetermined pressure differential,wherein each location corresponds to a respective element of the FEM;entering the data into the FEM; varying the tensor parameters of the FEMto obtain a patient-specific pressure response profile for the ONH ofthe patient; and using a computer to evaluate the profile in comparisonwith an empirical statistic to predict the onset of glaucoma.
 2. Amethod as recited in claim 1 wherein the measuring step furthercomprises the steps of: creating a first image of the ONH of the patientat a first pressure; creating a second image of the ONH of the patientat a second pressure; and assessing the first image and the second imageto establish the data.
 3. A method as recited in claim 2 wherein thepredetermined pressure differential is approximately 8 kPa.
 4. A methodas recited in claim 2 wherein the first pressure is approximately 2 kPa,and the second pressure is approximately 8 kPa.
 5. A method as recitedin claim 1 wherein the varying step is accomplished relative to a basecondition to simulate a change of pressure in the eye of the patientthrough a range of about 8 kPa.
 6. A method as recited in claim 5wherein the change of pressure is accomplished in pressure increments,wherein each pressure increment is equal to approximately 1 kPa.
 7. Amethod as recited in claim 1 wherein the FEM substantially replicates acylindrical shaped body having a first end surface and a second endsurface with a cylindrical surface therebetween, and further whereinelements of the FEM representing the PrNT are arranged on the first endsurface, wherein elements of the FEM representing the PoNT are arrangedon the second end surface, wherein elements of the FEM representing theLC are located between the first end surface and the second end surface,and wherein elements of the FEM representing the sclera are arranged onthe cylindrical surface.
 8. A method as recited in claim 7 wherein theelements of the FEM representing the sclera include a plurality of fiberelements, with the fiber elements transitioning in an outward directionfrom the cylindrical surface, with the transition being characterized bya change from a substantially circumferential orientation at thecylindrical surface to an increasingly spiral orientation withincreasing distance from the cylindrical surface to add stability to theFEM.
 9. A method as recited in claim 8 wherein the elements of the FEMrepresenting the LC include simulated pores, wherein a location and asize for each pore is determined by Second Harmonic Generation (SHG)imaging of the ONH.